A subquery is a query that is nested within another query. Subqueries let queries on one table dynamically adapt based on the contents of another table. This technique provides great flexibility and expressive power for SQL queries.

A subquery can return a result set for use in the FROM or WITH clauses, or with operators such as IN or EXISTS.

A scalar subquery produces a result set with a single row containing a single column, typically produced by an aggregation function such as MAX() or SUM(). This single result value can be substituted in scalar contexts such as arguments to comparison operators. If the result set is empty, the value of the scalar subquery is NULL. For example, the following query finds the maximum value of T2.Y and then substitutes that value into the WHERE clause of the outer block that queries T1:

SELECT x FROM t1 WHERE x > (SELECT MAX(y) FROM t2);

Uncorrelated subqueries do not refer to any tables from the outer block of the query. The same value or set of values produced by the subquery is used when evaluating each row from the outer query block. In this example, the subquery returns an arbitrary number of values from T2.Y, and each value of T1.X is tested for membership in that same set of values:

SELECT x FROM t1 WHERE x IN (SELECT y FROM t2);

Correlated subqueries compare one or more values from the outer query block to values referenced in the WHERE clause of the subquery. Each row evaluated by the outer WHERE clause can be evaluated using a different set of values. These kinds of subqueries are restricted in the kinds of comparisons they can do between columns of the inner and outer tables. (See the following Restrictions item.)

For example, the following query finds all the employees with salaries that are higher than average for their department. The subquery potentially computes a different AVG() value for each employee.

SELECT employee_name, employee_id FROM employees one WHERE
  salary > (SELECT avg(salary) FROM employees two WHERE one.dept_id = two.dept_id);

Syntax:

Subquery in the FROM clause:

SELECT select_list FROM table_ref [, table_ref ...]

table_ref ::= table_name | (select_statement)

Subqueries in WHERE clause:

WHERE value comparison_operator (scalar_select_statement)
WHERE value [NOT] IN (select_statement)
WHERE [NOT] EXISTS (correlated_select_statement)
WHERE NOT EXISTS (correlated_select_statement)

comparison_operator is a numeric comparison such as =, <=, !=, and so on, or a string comparison operator such as LIKE or REGEXP.

Although you can use non-equality comparison operators such as < or >=, the subquery must include at least one equality comparison between the columns of the inner and outer query blocks.

All syntax is available for both correlated and uncorrelated queries, except that the NOT EXISTS clause cannot be used with an uncorrelated subquery.

Impala subqueries can be nested arbitrarily deep.

Standards compliance: Introduced in SQL:1999.

Examples:

This example illustrates how subqueries can be used in the FROM clause to organize the table names, column names, and column values by producing intermediate result sets, especially for join queries.

SELECT avg(t1.x), max(t2.y) FROM
  (SELECT id, cast(a AS DECIMAL(10,5)) AS x FROM raw_data WHERE a BETWEEN 0 AND 100) AS t1
  JOIN
  (SELECT id, length(s) AS y FROM raw_data WHERE s LIKE 'A%') AS t2;
  USING (id);

These examples show how a query can test for the existence of values in a separate table using the EXISTS() operator with a subquery.

The following examples show how a value can be compared against a set of values returned by a subquery.

SELECT count(x) FROM t1 WHERE EXISTS(SELECT 1 FROM t2 WHERE t1.x = t2.y * 10);

SELECT x FROM t1 WHERE x IN (SELECT y FROM t2 WHERE state = 'CA');

The following examples demonstrate scalar subqueries. When a subquery is known to return a single value, you can substitute it where you would normally put a constant value.

SELECT x FROM t1 WHERE y = (SELECT max(z) FROM t2);
SELECT x FROM t1 WHERE y > (SELECT count(z) FROM t2);

Usage notes:

If the same table is referenced in both the outer and inner query blocks, construct a table alias in the outer query block and use a fully qualified name to distinguish the inner and outer table references:

SELECT * FROM t1 one WHERE id IN (SELECT parent FROM t1 two WHERE t1.parent = t2.id);

The STRAIGHT_JOIN hint affects the join order of table references in the query block containing the hint. It does not affect the join order of nested queries, such as views, inline views, or WHERE-clause subqueries. To use this hint for performance tuning of complex queries, apply the hint to all query blocks that need a fixed join order.

Internal details:

Internally, subqueries involving IN, NOT IN, EXISTS, or NOT EXISTS clauses are rewritten into join queries. Depending on the syntax, the subquery might be rewritten to an outer join, semi join, cross join, or anti join.

A query is processed differently depending on whether the subquery calls any aggregation functions. There are correlated and uncorrelated forms, with and without calls to aggregation functions. Each of these four categories is rewritten differently.

Column statistics considerations:

Because queries that include correlated and uncorrelated subqueries in the WHERE clause are written into join queries, to achieve best performance, follow the same guidelines for running the COMPUTE STATS statement as you do for tables involved in regular join queries. Run the COMPUTE STATS statement for each associated tables after loading or substantially changing the data in that table. See Table and Column Statistics for details.

Added in: Subqueries are substantially enhanced starting in Impala 2.0 for CDH 4, and CDH 5.2.0. Now, they can be used in the WHERE clause, in combination with clauses such as EXISTS and IN, rather than just in the FROM clause.

Restrictions:

The initial Impala support for nested subqueries addresses the most common use cases. Some restrictions remain:

• Although you can use subqueries in a query involving UNION or UNION ALL in Impala 2.1.0 and higher, currently you cannot construct a union of two subqueries (for example, in the argument of an IN or EXISTS operator).

• Subqueries returning scalar values cannot be used with the operators ANY or ALL. (Impala does not currently have a SOME operator, but if it did, the same restriction would apply.)

• For the EXISTS and NOT EXISTS clauses, any subquery comparing values from the outer query block to another table must use at least one equality comparison, not exclusively other kinds of comparisons such as less than, greater than, BETWEEN, or !=.

• Currently, a scalar subquery cannot be used as the first or second argument to the BETWEEN operator.

• A subquery cannot be used inside an OR conjunction. Expressions inside a subquery, for example in the WHERE clause, can use OR conjunctions; the restriction only applies to parts of the query "above" the subquery.

• Scalar subqueries are only supported in numeric contexts. You cannot use a scalar subquery as an argument to the LIKE, REGEXP, or RLIKE operators, or compare it to a value of a non-numeric type such as TIMESTAMP or BOOLEAN.

• You cannot use subqueries with the CASE function to generate the comparison value, the values to be compared against, or the return value.

• A subquery is not allowed in the filter condition for the HAVING clause. (Strictly speaking, a subquery cannot appear anywhere outside the WITH, FROM, and WHERE clauses.)

• You must use a fully qualified name (table_name.column_name or database_name.table_name.column_name) when referring to any column from the outer query block within a subquery.

• The TABLESAMPLE clause of the SELECT statement does not apply to a table reference derived from a view, a subquery, or anything other than a real base table. This clause only works for tables backed by HDFS or HDFS-like data files, therefore it does not apply to Kudu or HBase tables.

Complex type considerations:

For the complex types (ARRAY, STRUCT, and MAP) available in CDH 5.5 / Impala 2.3 and higher, the join queries that "unpack" complex type columns often use correlated subqueries in the FROM clause. For example, if the first table in the join clause is CUSTOMER, the second join clause might have a subquery that selects from the column CUSTOMER.C_ORDERS, which is an ARRAY. The subquery re-evaluates the ARRAY elements corresponding to each row from the CUSTOMER table. See Complex Types (CDH 5.5 or higher only) for details and examples of using subqueries with complex types.

Related information:

EXISTS Operator, IN Operator